It’s been called the most important 26 seconds of film in history: The 486 frames of 8-millimeter Bell + Howell home movie footage shot in the midday sun of Dallas on November 22, 1963, by a dressmaker named Abraham Zapruder. Twenty-six seconds that included a historic, horrific, all-too-clear vision of a presidential assassination.

Most people vaguely know about the Zapruder film, but it will soon become omnipresent as the 50th anniversary of the assassination of John F. Kennedy approaches. What is not well known, however, is that a single frame of it was kept largely secret from public view for 12 years after the assassination.

Frame 313. The frame that gave Abraham Zapruder nightmares, the frame he insisted be withheld from the public—a single frame of film that can be said to have changed American history and culture.

“We like to feel that the world is safe,” Errol Morris tells me. “Safe at least in the sense that we can know about it. The Kennedy assassination is very much an essay on the unsafety of the world. If a man that powerful, that young, that rich, that successful, can just be wiped off the face of the earth in an instant, what does it say about the rest of us?”

That instant is one we can all now watch on YouTube. In fact, there is a YouTube compilation that includes no fewer than five versions of the Zapruder film—slow-motion, zoomed-in, close-ups. Once you’ve seen the unspeakable act it captures, Morris says, your sense of stability and safety, your sense of the rationality of the world, has been forever lost. It’s “endlessly haunting and disturbing,” he says.

I wanted to talk to Morris about the Zapruder film because as a documentary filmmaker he’s focused on mysteries (he freed an innocent man from death row with The Thin Blue Line); he’s re-examined secret history (he won an Oscar for cross-examining the enigmatic Robert McNamara in The Fog of War). As a writer, he’s questioned the way that photographic images can document the nature of truth (in his recent book Believing Is Seeing).

So one recent morning over breakfast in the dining room of the hotel in New York’s SoHo where Morris was staying, I sat down in front of his computer to watch the Zapruder film with him.

***

The Zapruder film, silent but in color, shows a motorcade led by two open-top limousines proceeding at a stately pace through a street lined with people. We see the black Lincoln Continental with JFK and his wife, Jackie, in a pink skirt-and-jacket suit. We see them all waving to onlookers as the motorcade heads for what was known locally as “the triple underpass,” an aptly metaphoric name for the tangle we will soon enter.

As the motorcade approaches, we see JFK’s car emerge from behind a sign that had been temporarily blocking the view. Suddenly, we see JFK clutch his throat. Jackie leans over to attend to him. An instant later, in Frame 313, it looks like a lightning bolt strikes JFK’s head. We see it blown up and thrown back. Jackie frantically crawls over the rear seat of the open car and climbs onto its rear deck grasping at something that has been described as a piece of her husband’s shattered skull. If Frame 313 is the forensic peak of the Zapruder film, this sight is the almost-unbearable emotional heart of it.

Rewind to Frame 313: The visceral impression that the blast came from in front of JFK and blew his head backward is powerful. There have been arguments that this is a kind of optical illusion—the most convincing to me being that JFK had been hit from behind after the previous frame, 312, slamming his chin forward to his chest, and his head was rebounding backward in Frame 313.

And it would be so much easier to dismiss the impression of a frontal shot as an illusion, because otherwise you’d have to doubt the conclusion of the Warren Commission that Lee Harvey Oswald, who was positioned behind the president, was the lone gunman.